The invention pertains to machine tools such as may be used in a machining center.
An example of a machining center of this type is shown in FIG. 1 of the accompanying drawings. In FIG. 1, reference numeral 1 identifies a bed which acts as a supporting base. On top of the bed 1 are provided a pair of guide rails 3 extending in the longitudinal (X) direction. Slidably mounted on the guide rails 3 is a table 4. The table 4 may be reciprocated in the X direction by a drive mechanism (not shown). A second pair of guide rails 5 are mounted on the table 4, extending in the transverse (Z) direction. A column member 6, slidably mounted on the guide rails 5, is reciprocatable in the transverse direction by a second drive mechanism (not shown). A window 7 is formed in the front surface of the column member 6, and a third pair of guide rails 8 are provided in the window 7, extending vertically. A spindle head 2, slidably supported by the third pair of guide rails, is reciprocatable in the vertical (Y) direction by a third drive mechanism (not shown). A tool 10, for instance, a drill, driven by a motor 9, is removably mounted on the spindle head 2.
A jig bench 11 is provided at the front of the bed 1. A workpiece (not shown) to be machined by the tool 10 is secured by a jig to the jig bench 11. A discharge passage 12 is formed below the jig bench 11 to collect and discharge material such as metal chips produced during a machining operation.
In a machining operation, the tip of the tool 10 is properly positioned relative to the workpiece on the jig bench 11 by moving the table 4 in the X direction, the column member 6 in the Z direction, and the spindle head 2 in the Y direction by the associated drive mechanisms.
During machining, a reaction force F is produced by the pressure of the tool 10 against the workpiece. Especially, if the spindle head 2 is positioned near the upper end of its range, the force F produces a strong moment acting on the guide rails 3.
In the aforementioned machine tools, a force applied to portions where the table 4 is engaging the guide rails 3 is the sum of reaction of weights of the table 4, the column member 6 and reaction of moment derived from the external force. Under the above condition, a great deal of force is created where the distance between of the tool 10 and the guide rails 3 is long. This tends to cause deformation of the table 4, and hence a reduction in the machining accuracy. It is of course possible to alleviate this problem somewhat by making the table 4 thicker. Doing so, however, is accompanied by the further disadvantage that the machine is made heavier and its cost increased.